Process for removing surface distortion from a metal article

ABSTRACT

A process to remove a surface distortion in a metal article, example of which is a landing gear shock strut web. To remove such a distortion, commonly called a &#39;&#39;&#39;&#39;can&#39;&#39;&#39;&#39; in industry, the portion of the article surrounding the &#39;&#39;&#39;&#39;can&#39;&#39;&#39;&#39; is clamped on both of its sides with a pressure just under the maximum compressive strength of the material. Heat is applied to the restrained affected region, such as through heated platens. The clamping and restraining pressure is maintained until the metal reaches a preset temperature at which the region &#39;&#39;&#39;&#39;thermally expands&#39;&#39;&#39;&#39; within itself. The pressure and the heat are then removed from the metal article, and the article is cooled. Upon cooling, the distortion or &#39;&#39;&#39;&#39;can&#39;&#39;&#39;&#39; is thereby removed, and the metal article is acceptable for its intended purpose. Chilling of the article prior to heating provides an advantageous benefit so that the maximum change in temperature of the affected region is obtained after the part is clamped.

United States Patent [191 Sparling PROCESS FOR REMOVING SURFACE DISTORTION FROM A METAL ARTICLE [75] Inventor: Kenneth P. Sparling, Burbank,

Calif.

[73] Assignee: Lockhead Aircraft Corporation,

' Burbank, Calif.

22 Filedz Aug. 6, 1973 21 Appl. No.: 386,164

[52] US. Cl. 72/342, 148/131 -Primary Examiner-Lowell A. Larson Attorney, Agent, or FirmBilly G. Corber; Frank L. Zugelter; Lowell G. Turner [451 Dec. 10, 1974 [5 7] ABSTRACT A process to remove a surface distortion in a metal article, example of which is a landing gear shock strut web. To remove such a distortion, commonly called a can in industry, the portion of the article surrounding the can is clamped on both of its sides with a pressure just under the maximum compressive strength of the material. Heat is applied to the restrained affected region, such as through heated platens. The clamping and restraining pressure is maintained until the metal reaches a pre-set temperature at which the region thermally expands" within itself. The pressure and the heat are then removed from the metal article, and the article is cooled. Upon cooling, the distortion or can is thereby removed, and the metal article is acceptable for its intended purpose. Chilling of the article prior to heating provides an advantageous benefit so that the maximum change in temperature of the affected region is obtained after the part is clamped.

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PROCESS FOR REMOVING SURFACE DISTORTION FROM A METAL ARTICLE BACKGROUND OF THE INVENTION 1. Field of the Invention The field of art to which the invention is most likely to pertain is located in a class of processes generally relating to metal treatment, and metal reforming and deforming. Classes 72, Metal Deforming; and 148, Metal Treatment, US. Patent Office Classifications, may be the applicable general areas of art in which the claimed subject matter of the type involved here may be classified.

2. The Prior Art Examples of prior art devices in the arts to which this invention most likely pertains are US. Pat. Nos. 2,599,786; 3,094,160; 3,130,088; and 3,342,049.

PROBLEMS EXISTING IN THE PRIOR ART The failure of mechanical straightening methods for utilization in industry upon thinner areas in large, heavy, forged, machined and/or otherwise fabricated metal components, such as aircraft landing gear shock struts, or flat or contoured sheet metal parts, has been a primary cause for scrapping these parts (components). However, in some limited cases, such as with shock struts, the-portion of such an article containing the unstraightened or distorted portion, such as a web in a pocket of the shock strut, would be cut out by machining, and thereafter a critically-machined flat sheet of metal would be fabricated for insertion and mechanical fastening in the opening formed by the removal of the distorted region. This, of course, led to an increase in weight and considerable increase in cost of the article or component, although less expensive than scrapping the entire component and completely fabricating a new one and to which the possibility of distortion again applied.

Quite often, large forgings have as much as 80 percent of the forged material, such as aluminum, machined' away, leaving thin webs which would experience warpage, severe or otherwise, commonly referred to as cans. In regard to landing gear shock struts, these cans" are caused by residual stresses in the forging induced by forging, heat treatment, machining or other causes, although for purposes of this invention, the particular reason for a can which does exist is immaterial. A can in such a forging web makes unacceptable such a component for its intended purpose, and it would have to be scrapped, discarded, or machine repaired as noted above. The ascertainment of an existing can, after completion of machining the forging or before utilization of an article or shock strut having a nominally flat surface, is accomplished by visual observation and inspection, with the assistance of a straight edge and feeler gage. The straight edge is applied across the nominally flat surface of the article and on both of its sides. As the straight edge is drawn across the flat surface on one side of the article, an air gap appears along or under the length of the straight edge, thereby showing that a distortion or can exists in the article, The degree of deformation may be measured by sliding an appropriate thickness of the feeler gage under the straight edge. Of course, this same techniquewith the straight edge may be used across the same flat surface or surfaces after this process has been carried out, to show that such distorted area no longer exists.

Thus, the purpose of this invention is to flatten or straighten those surface portions of a nominally flat metallic article which are distorted, and thereby make the article once again acceptable for use. Such a distortion occurs in a local or single region, with excess material forming a convex dome-like appearance in the article or a concave dish-like appearance on the opposite side, and which is commonly referred to as a can. It should be understood, however, that the process is not limited in its application to nominally flat surfaces, but is also adaptable to curvilinear situations. Nor need the metal article being treated be limited to one already having a heat treatment, but the process can be utilized on a non-heat treated metal article as well. It should be noted, however, that in the case of the process being applied to a heat treated metal article, that the highest temperature to which the can area is subjected or exposed to, should remain below the final aging or tempering temperature that is utilized in the manufacture of the article.

SUMMARY OF THE INVENTION This invention pertains to a method for removing surface distortions in a metal article, and is particularly related to a mechanical/thermal activity or process by which the metal itself is caused to be flattened or returned to its original or desired configuration.

It is particularly adaptable to that type of distortion that is within the elastic range of the metal from which the article is fabricated, that is, to the type of distortion that may be completely removed while subjected to mechanical or other external pressure and will return to the full original degree of distortion upon removal of the external pressure due to the elastic properties of the metal.

An object of this invention is to work or repair a distorted metal article so that it resumes its orignal or desired shape or configuration.

Another object of this invention is to eliminate additional work and/or machining otherwise required to repair warpage or other distorted surfaces in a metal article.

Another object of this invention is to eliminate the scrapping or discarding of a metallic article unfit for its intended purpose.

A further object of this invention is to repair a heattreated metallic article without destroying the advantages of the heat treatment provided such article.

Another object of this invention is to provide a reduction in surface area or lengths in two dimensions which a distorted metallic article would otherwise assume, by thermally compressing a local region of such article so that a shape or configuration in the same general plan as the rest of the article, or workpiece may be obtained in such region.

A still further object of this invention is to reduce expense and amount of metallic materials that are otherwise involved in fabricating large and heavy metallic components for articles, machines and the like, such as a landing gear shock strut for an airplane.

Another object of this invention is to eliminate the need for reheat treatment.

These and other objects and advantages of the invention will become more apparent upon a full and complete reading of the following description, its appended claims, and the drawing accompanying this specification and comprising three sheets.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective schematic illustration ofa distorted metal article undergoing application of this novel process by which its distortion is to be removed.

FIGS. 2, 3 and 4 schematically represent the carrying out of the process by which such a distortion is removed.

FIGS. 5, 6 and 7 illustrate a modified version of apparatus that may be utilized in carrying out the process.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing in which reference characters correspond to like numerals hereinafter following, a metal article 14 (FIG. 1) with a distorted portion or can 16 located upon one of its surface dimensions is positioned between a pair of opposed clamping members 19, 21. To date, the clamping members 19, 21 have best taken the form of thin sheet metal clamping rings 23, 25 for purposes of repairing landing gear shock struts. In the illustration of FIG. 1, the clamping rings 23, 25 comprise one-piece elements with turned flanges for mounting, each including a recess or opening 24 into which the can 16 becomes disposed upon the clamping together of the members 19, 21 on the article 14. The clamping members 19, 21 are mounted on and suitably secured to respective platens 26, 27 which are to be heated for the operation and carrying out of the process. Adequate pairs of lead wires 30, 31 provide electrical energy to heat the platens 26, 27 respectively. The upper platen 26 is suitably secured in a press mounting, while the lower platen 27 is secured upon a base member (not shown) of the same press, such as an hydraulic gap (C-type) press. The clamping members 19, 21 are fastened to their corresponding platens 26, 27 in any suitable, convenient way.

The heated clamping members 19, 21, together with their corresponding heated platens 26, 27 are brought to bear on the metal article 14 in such a manner that' the opening 24 bears the distorted portion 16. Thus, the marginal contact area on the metal article 14 immediately surrounding the surface distortion 16 is clamped securely to confine lateral motion of the portion being treated to the region within the clamping rings 23 and 25.

The platens 26, 27 are heated via the lead wires 30, 31 which are connected to cartridge heaters (not shown, but well known in the arts) which in turn are connected to a suitable power and temperature control apparatus (not shown) which provides the correct volt age and current in proper proportions for converting electricity to heat. This heat is conducted from such heaters directly to and through the platens 26, 27, and thence to the metal article 14 upon clamping the members 19, 21 thereto.

A suitable pressure is quickly applied to the article 14 through the clamping members 19, 21, and the platen faces within the openings 24 upon the can", by means of the press in which these members are mounted. The clamping pressure is quickly increased and maintained at a suitable magnitude until the temperature of the article 14 reaches a predetermined or pre-set value. At

that time, the clamping pressure and the heat are removed from the article 14, and the article is cooled, such as by the ambient temperatures surrounding the area of work, or by quenching, chilling, with solid CO or other suitable ways.

It has been found in actual operation upon shock struts that the can is removed. It is my understanding that these cans" are caused by residual stresses developed in the fabrication of the shock struts. In carrying out the process, the heat which is applied to a strut causes a portion thereof lying within the clamping members 19, 21 and in and about the distorted surface 16, to thermally expand. The clamping restrains such local thermal expansion from progressing outside the periphery of the clamping forces. As the canned area expands, the opening 24 provides for a lateral or horizontal expansion to occur for and within the article 14, but restricts or prevents a vertical motion for the article 14. The thermal expansion forces must take the direction shown by the arrows 33 illustrated in FIG. 3, and which occur within the metal article 14 during the heating of the article 14. These thermal expansion forces exceed the yield strength of the material and in effect cause compression of the material upon itself and within the clamped area provided by the members 19, 21. When the clamping pressure and accompanying heat are removed from the article 14, the temperature thereof decreases. Upon cooling, the can no longer exists.

In cases where a severe distortion occurs in the metallic article, it has been found that the operation may be repeated as many times as necessary to remove the can".

FIGS. 2, 3 and 4 illustrate that the process may be carried out by incorporating a shallow machined relief 40 directly into a pair of platen members 45, 47. The members 45, 47 are clamped upon an article 48, around and upon its distorted portion 49, and are caused to heat the article 48. Upon removing the clamping pressure and heat, the article 48 is repaired, as shown in FIG. 4.

FIGS. 5, 6 and 7 illustrate further that the process may be carried out in other than the form of pressure apparatus suggested above. A first clamping member 55, in the form of, say, a sleeve, is caused to relatively descend toward a corresponding or mating clamping member 57, after the heating of such members 55, 57 and after the insertion of an article 58 having a canned portion 59 within the confines of such members 55, 57, until the article 58 is contacted. A heated piston 60 is caused to rapidly slide or be pressed onto the canned portion 59, and is guided by the first clamping member 55 and a suitable flattening pressure is applied. FIG. 7 represents schematically what may be considered as the initial displacement of the distorted region 59, as the piston 60 begins to mechanically act thereupon. I.e., the piston 60 has not yet seated in its final translation downwardly. After the process has been carried out, the article 54 is in repaired condition as illustrated in FIG. 4.

A particular step in addition to those described above has been found to be very advantageous and beneficial to the efficacy of the process. And that is the chilling of the metal article immediately prior to the imposition of the heating and clamping steps. The greater the temperature difference between that temperature attained in the heating step and the temperature of the article just prior to applicaton of clamping pressure to the area on which the can is to be removed the more effective the straightening of the article or removal of the can.

A particular example of carrying out the process will now be described, as applied to making acceptable once again a distorted airplane landing gear shock strut web such as used in the airplane model L-lOl l manufactured by the assignee. The strut itself was forged and machined out of aluminum 7075T6, and inspection prior to installation in the aircraft assembly line ascertained that a can had developed therein.

In removing such a can, 302 stainless steel faced 7075 aluminum clamp blocks or platens 26, 27 were utilized. A pair of 0.005 inch thick, /2 inch wide, 302 stainless steel flanged clamp rings 23, 25 were formed upon the faces of the platen blocks 26, 27 so as to provide for the opening 24. Initially then, the platen blocks, mounted in a C-type press, were heated to 325F, at which temperature the press was ready to accept the distorted strut.

Immediately prior to inserting the strut into its position in the press, the portion thereof (a machined pocket) which was deformed and the two adjacent machined pockets were chilled to approximately F, by applying solid CO (dry ice) directly to such pockets or areas in order to achieve the maximum straightening for each pressing. By observation, it was seen that a frost coating formed on this area of the shock strut. This meant that the temperature of the strut was certainly below freezing temperature. In the instant case of repair, it was not necessary in carrying out the process to measure or record the actual reduction in the temperature of such strut, although it is believed that these steps are readily known and available to the mechanic skilled in the art.

To continue, the strut is transferred to the press as quickly as possible, and the press is closed rapidly to clamp the periphery of the can", using, say, 150 tons of force in such clamping step. The clamping ring 23 preferably engages the circumferential margin immediately adjacent the *can". In this instance of the practice of the invention, it should be noted that the pressure and heat are applied substantially simultaneously, if not simultaneously.

In this particular example being considered, the unit clamping pressure upon a 5 inch X 8 /2 inch rounded corner machined pocket containing the can" approximates 25,000 psi. This force is close to the compressive yield strength of the 7075-T6 aluminum material from which the strut was fabricated, where this yield strength is based upon the mechanical properties of the material while at the straightening temperature.

The canned area was-held under this clamping pressure until such area reached a temperature of about 300325"-F. A period of time approximating 2-3 minutes wasrequired.

Thereafter, both the clamping and heating were removed, and the strut allowed to cool in the ambient environment. In another application of the process to a similar type shock strut web; solid CO was used to cool the article. In each case the can no longer existed.

It should be noted that the clamping step requires complete, full and continuous pressure throughout or around the periphery of the canned area. Should such periphery not be flat, shimming or contouring of the face of the clamping blocks is one way by which such uninterrupted peripheral pressure may be acquired.

Also, the stainless steel clamp ring members can be fabricated of different widths, depending upon the peripheral length of the clamped can area, so that the clamping pressure is just under the yield strength of the workpiece (strut) at the temperature (325 "F in the case of the above example) at which the yield strength is measured. In this manner, a constant pressure press is adaptable to the process, without coining the article being treated, or a larger area may be straightened with a limited tonnage press capacity.

In the example cited above, a ton hydraulic gap press, with a C frame, and manufactured by Rodgers Hydraulic, Inc., 7401 Walker St., Minneapolis, Minnesota, was utilized. Also, cartridge heaters, manufactured by General Electric Company, Schenectaday, New York, provided the means to convert the required electricity to heat, from a 220 volt source.

Thus, it will be seen that this novel method eliminates the scrapping or discarding of parts heretofore never thought to be salvageable, and thereby provide for acceptable parts without an excessive cost to do so. This particular method is very advantageous in cases where the heat treat of the metal undergoing such process is not to be disturbed. This, of course, is very important in qualifying large metallic components for aircraft.

It should now be apparent that the power of thermal expansion causes compression or shrinking of the canned area, as the process is carried out. This contradiction of obtaining compression by thermal expansion is obtained through the use ofa rigid continuous clamping around the distorted area. The clamping restrains any tendency for the metal material in the distorted area to move outside of the clamp assembly upon the metal article.

In addition to the clamping assembly providing a total restraint in any lateral direction for the metal article within the confines of such assembly, the face of the clamping members, being vertically relieved, as at 24, provides a free lateral motion for the material side of the clamped area. This relief must be small enough to prevent undue additional vertical distortion, and yet force the expansion of the material to compress or shrink the can. Immediately after release of the clamping pressure, the can may be higher than before the operation began, due to the local heating thereof. However, on cooling the article to a uniform temperature, the can is reduced or removed.

It is believed that this process works best on metal thicknesses in the sheet gage range, however, it is also believed that the process is applicable to thicker metal or plate materials provided heavy presses and large tools are available.

Pursuant to the requirements of the patent statues, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art to which it pertains, or with which it is most nearly connected, such exemplification including what is presently considered to represent the best embodiment of the invention. However, it should be clearly understood that the above description and illustrations are not intended to unduly limit the scope of the appended claims, but that therefrom the invention may be practiced otherwise than as specifically described and exemplified herein, by those skilled in the art, and having the benefit of this disclosure.

Therefore, what I claim as patentably novel is:

l. A method for removing surface distortion from a metal article, the method utilizing a pair of opposed clamp members having a marginal region surrounding a recess between mating surfaces thereof comprising the steps of:

heating the opposed clamp members;

positioning the metal article between the heated clamp members;

closing the clamp members upon the metal article under a clamping pressure and such that, at least a portion of the surface distortion is positioned within the recess and caused to be heated to a preset value,

opening the clamp members; and

cooling the metal article.

2. The method of claim 1 wherein the step of closing the clamp members is done quickly.

3. The method of claim 1 including the step of chilling the metal article prior to the clamping step.

4. A method for removing a surface distortion from a metal article comprising the steps of,

- restraining an area immediately surrounding the surface distortion from lateral movement beyond the restraint,

heating the surface 'distortion to a pre-set temperature,

pressing the distorted area, and cooling the metal article.

5. The method of claim 4 including the step of chilling the metal article prior to the restraining step.

6. The method of claim 4 wherein the heating step comprises transferring the heat from a pair of opposed preheated platens clamped upon the metal article about the surface distortion.

7. The method of claim 6 including the step of chilling the metal article prior to the restraining step.

8. A method for removing a surface distortion from a metal article comprising the steps of:

restraining the metal article in a region marginally surrounding at least a portion of the surface distortion so as to preclude lateral motion beyond the restrained region of the metal material within such region;

subjecting the metal within the restrained region to an elevated temperature sufficient to result in its thermal expansion;

releasing the restraint; and cooling the metal article.

is chilled prior to the restraining step. 

1. A method for removing surface distortion from a metal article, the method utilizing a pair of opposed clamp members having a marginal region surrounding a recess between mating surfaces thereof comprising the steps of: heating the opposed clamp members; positioning the metal article between the heated clamp members; closing the clamp members upon the metal article under a clamping pressure and such that, at least a portion of the surface distortion is positioned within the recess and caused to be heated to a pre-set value, opening the clamp members; and cooling the metal article.
 2. The method of claim 1 wherein the step of closing the clamp members is done quickly.
 3. The method of claim 1 including the step of chilling the metal article prior to the clamping step.
 4. A method for removing a surface distortion from a metal article comprising the steps of, restraining an area immediately surrounding the surface distortion from lateral movement beyond the restraint, heating the surface distortion to a pre-set temperature, pressing the distorted area, and cooling the metal article.
 5. The method of claim 4 including the step of chilling the metal article prior to the restraining step.
 6. The method of claim 4 wherein the heating step comprises transferring the heat from a pair of opposed preheated platens clamped upon the metal article about the surface distortion.
 7. The method of claim 6 including the step of chilling the metal article prior to the restraining step.
 8. A method for removing a surface distortion from a metal article comprising the steps of: restraining the metal article in a region marginally surrounding at least a portion of the surface distortion so as to preclude lateral motion beyond the restrained region of the metal material within such region; subjecting the metal within the restrained region to an elevated temperature sufficient to result in its thermAl expansion; releasing the restraint; and cooling the metal article.
 9. The method of claim 8, wherein the metal material within the restrained region is restricted in its movement normal to said lateral motion.
 10. The method of claim 8, wherein the heat for said obtaining said elevated temperature is provided through heat transfer from preheated tooling utilized in restraining the metal article.
 11. The method of claim 8, wherein the metal article is chilled prior to the restraining step. 